1. Field of the Invention
The present invention relates generally to acoustic apparatus, and more particularly, to an apparatus for providing an audio signal with predetermined signal processing to be supplied to a speaker so that reproduced sound based on the audio signal is obtained from the speaker functioning as a sound source.
2. Description of the Prior Art
There have been proposed various kinds of acoustic apparatus operative to reproduce sound based on an audio signal. When reproduced sound representing music, speech or the like is obtained by one or more of the acoustic apparatus to be heard by a listener, it is generally desired that a sound source generating the reproduced sound is positioned in front of the listener. This is easily understood in view of the ordinary fact that a listener tends naturally to take his or her position to face a sound source in a limited sound field, such as a space in a hall or the like, if it is unnecessary for the listener to move with the intention of some particular purpose or to be particularly restricted in position or direction of his or her face in the limited sound field.
Accordingly, for example, in the case where sound reproduced by a speaker or plural speakers which are supplied with audio signals is appreciated, such a layout that listeners are positioned to face the speaker functioning as a sound source or a main one of the plural speakers is established. With the layout thus established, each of the listeners can be mentally composed and stabilized.
Further, it is also desired that the sound source generating the reproduced sound is positioned at a listener's eye level in height. That is, in a positional relation in height between the sound source and the listener, the sound source positioned at listener's eye level contributes to causing the listener to be mentally composed and stabilized.
Under the situation mentioned above, when reproduced sound is obtained from a speaker or plural speakers provided for one or more acoustic apparatus to be heard by a listener, a condition wherein the listener is restricted in position, direction of his or her face, posture and so on is often brought about. In such a case, the listener is compelled to be put in a condition wherein the speaker functioning as a sound source is not positioned in front of the listener or is positioned at a level in height apparently different from a listener's eye level.
For example, in the case where sound reproduced by an acoustic apparatus employed in a vehicle (hereinafter, referred to an acoustic apparatus for vehicles) is enjoyed in a cabin of the vehicle, a listener, namely, a driver or other passenger of the vehicle who keeps usually his or her posture to sit down on a seat in the cabin is basically restricted in position, direction of his or her face, posture and so on to listen to the reproduced sound even though the seat is adjustable to some degree in its position and direction. In such a case, since the vehicle is usually furnished with various kinds of equipments each occupying a space limited at maximum in the cabin, a main portion of the acoustic apparatus for vehicles is often provided on a dashboard or a console shelf forming a front end portion of the cabin and speakers connected electrically with the main portion of the acoustic apparatus for vehicles are provided, for example, in lower portions of left and right doors attached to left and right side portions of a body of the vehicle, respectively.
The speakers thus provided in the lower portions of the left and right doors, respectively, are positioned neither in front of the driver or other passenger nor at the eye level of the driver or other passenger. Therefore, the driver or other passenger is compelled to listen to the reproduced sound from the speakers each functioning as a sound source, each of which is positioned at the left to right side or at the back of the driver or other passenger and at a level in height apparently different from the eye level of the driver or other passenger.
There has been previously proposed an improved acoustic system with which one or more speakers function as actual sound sources for generating reproduced sound and a listener can listen to the reproduced sound as if the reproduced sound is obtained from a position different from the positions of the speakers, for example, in front of the listener, in other words, the listener can recognize a virtual sound source at the position different from the positions of the speakers, as shown in, for example, Japanese patent laid-open publication No. HEI 5-316599. It would be possible to solve the aforementioned problems relative to the acoustic apparatus for vehicles if such an improved acoustic system as described above could be favorably applied to the acoustic apparatus for vehicles.
However, it is necessary to provide a circuit structure containing a compensating filter, a filter characteristic of which must be selectable, for causing audio signals to be subjected to predetermined signal processing and then to be supplied to the speakers when the aforementioned proposal of the improved acoustic system is intended to be put into practice and the circuit structure which contains the compensating filter to be provided for putting the improved acoustic system into practice brings about enlargement in scale of the circuit structure and considerable increase in cost. Accordingly, it seems to be quite difficult to apply favorably the improved acoustic system proposed previously to the acoustic apparatus for vehicles.